Rotary electrothermal actuator

ABSTRACT

An electrothermal actuator for converting electrical energy to rotational mechanical motion includes a housing, a flexible diaphragm disposed in the housing and defining a boiler chamber within the housing, a working fluid disposed within the boiler chamber and changing phase in response to heating, thereby displacing the diaphragm, a heater disposed within the boiler chamber for heating the working fluid in response to an electrical current flowing through the heater, and a rotatable shaft assembly partially disposed within the housing and including a rotating shaft extending outside the housing, rotating between a return position and a rotated position, and a driver for rotating the shaft from the return position toward a rotated position in response to heating of the working fluid, displacing the diaphragm.

This application is a division of application Ser. No. 07/676,630, filedSep. 30, 1991, now U.S. Pat. No. 5,203,171.

BACKGROUND

Electrothermal fluid displacement actuators that convert electricalenergy into thermal energy and, in turn, employ the thermal energy toexpand a thermally expandable fluid medium and produce linear motion areknown. Examples of such actuators are described in commonly assignedU.S. Pat. Nos. 4,029,941 and 4,070,946, the disclosures of which areincorporated herein by reference. Additional examples of electrothermalactuators are disclosed in U.S. Pat. Nos. 4,070,859, 4,079,589,4,104,507, 4,759,189, and 4,887,429, all commonly assigned.

In the electrothermal actuators described in the cited patents, aworking fluid is contained within a boiler chamber in the actuator.Preferably, that fluid is a liquid at room temperature but, upon heatingto a sufficient temperature, changes from a liquid phase to a gas phase.The phase transition results in an increasing pressure within the boilerchamber. The expanding working fluid presses against a diaphragm anddisplaces and/or expands the diaphragm to slide a piston within theactuator and a pin attached to the piston outwardly from the actuator.The out-stroking motion of the piston may be employed to actuate anexternal device. A return spring mounted within the actuator biases thepiston to retract the pin when the pressure within the boiler chamberdecreases. As long as the pressure of the working fluid is sufficient toovercome the biasing force of the return spring, the pin remainsextended from the actuator. When the working fluid cools and returns toa liquid phase, the pressure on the diaphragm is reduced, the diaphragmcontracts and/or retracts, and the piston and pin retract under theinfluence of the biasing force applied by the return spring.

In order to heat and produce a phase change in the working fluid, anelectrically-driven heater is disposed within the boiler chamber. Theheater may be a simple resistance heater that produces heat in responseto a current flow through the heater. The heater may be a positivetemperature coefficient (PTC) heating element that significantlyincreases in resistance once a particular temperature is reached. A PTCheater inherently limits the magnitude of the steady state current thatflows in response to a particular voltage applied to the heater. Toimprove actuator response time without excessive current flows, aresistance heater may be connected in series with a PTC heating element.

In all of the thermal actuators described in the cited patents, asliding piston extends and retracts a pin in response to an electricalsignal applied to the heater. In many applications, it is desirable toprovide a rotary motion rather than the linear motion produced by theknown actuators.

SUMMARY OF THE INVENTION

Accordingly, it is a primary object of the invention to provide anelectrothermal actuator that produces a rotary motion of a mechanicalelement in response to an electrical signal.

Another object of the present invention is to provide an electrothermalactuator that is small in size, uses few parts, and is low in cost forproviding a rotary mechanical output in response to an electrical input.

According to the present invention, a rotary electrothermal actuatorincludes a housing, a flexible diaphragm disposed within the housing anddefining a boiler chamber, a working fluid disposed within the boilerchamber, a heater disposed within the boiler chamber for heating theworking fluid in response to a current flow through the heater, arotating shaft assembly disposed partially within and partially outsidethe housing and including a rotating shaft extending outwardly from thehousing rotating between a return position and a rotated position, andmeans for rotating the shaft from the return position to a rotatedposition in response to heating of the working fluid that displaces thediaphragm.

In one embodiment, the rotating shaft assembly and the means forrotating includes a threaded lead screw attached to the rotating shaftand disposed within the housing and a piston nut including threadscomplementary to and engaging the lead screw threads. The piston nut isprevented from rotating within the housing and is in contact with thediaphragm. Sliding displacement of the piston nut by the diaphragmcauses rotation of the lead screw that threadedly engages the pistonnut, thereby rotating the shaft.

In another embodiment, the rotating shaft assembly and means forrotating include a cam driver and cam disposed within the housingincluding opposing faces having variable depth recesses and a sphericalmember contacting and disposed within the opposed recesses in the camdriver and cam. The cam driver is prevented from rotating within thehousing and is in contact with the diaphragm. Sliding displacement ofthe cam driver causes rotation of the cam to which the shaft is attachedthrough a rolling action of the spherical members within the variabledepth recesses.

Other objects and advantages of the present invention will becomeapparent from the detailed description given hereinafter. It should beunderstood, however, that the detailed description and specificembodiments are given by way of illustration only, since various changesand modifications within the spirit and scope of the invention willbecome apparent from the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of an embodiment of a rotary electrothermalactuator according to the invention.

FIGS. 2(a) and 2(b) are a sectional side view and a front end view of anembodiment of a rotary electrothermal actuator according to theinvention with the shaft in the return position.

FIGS. 3(a) and 3(b) are a sectional side view and a front end view of anembodiment of a rotary electrothermal actuator according to theinvention with the shaft in the rotated position.

FIG. 4 is a rear end view of an embodiment of an electrothermal actuatoraccording to the invention.

FIGS. 5(a) and 5(b) are a sectional side view and a shaft end view of analternative embodiment of a rotary electrothermal actuator according tothe invention with the shaft in the return position.

FIGS. 6(a) and 6(b) are a sectional side view and a shaft end view of analternative embodiment of a rotary electrothermal actuator according tothe invention with the shaft in the rotated position.

FIGS. 7(a) and 7(b) are frontal views of alternative cam driver and camfaces that may be employed in embodiments of the invention.

FIG. 8 is a sectional side view of still another embodiment of anelectrothermal actuator according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

In FIG. 1, a side view of a rotary electrothermal actuator 1 accordingto one embodiment of the invention is shown. The actuator 1 includes ahousing 2, preferably metal, including a rear housing 3 and fronthousing 4 that are joined together by a crimp seal at a collar 5. Twoelectrical leads 6 and 7 protrude from the rear housing 3 for supplyingan electrical current to actuate the actuator 1, i.e., to rotate a shaft10 that protrudes from the front housing 4. The electrical leads 6 and 7are seen more clearly in FIG. 4, a view from the rear end of theactuator. Front views of the actuator in normal and actuated positions,respectively, are shown in FIGS. 2(b) and 3(b), respectively. FIGS. 2(a)and 3(a) are sectional views with the internal elements of the actuatorin positions respectively corresponding to the front views of FIGS. 2(b)and 3(b).

In FIG. 2(a), the rear housing 3 encloses a boiler chamber 20 containinga working fluid that is liquid at the normal ambient temperature butchanges to a gaseous phase upon heating. A heater 21, which may be aresistance heater or a PTC heater, is housed within the boiler chamber20 and includes leads 22 and 23 respectively connected to electricalleads 6 and 7 after passing through a plastic, resinous, ceramic, glassor other electrically insulating feedthrough 24 that closes the rear endof the rear housing 3. At the front end of the boiler chamber 20, aflexible elastic diaphragm 25 defines the front wall of the boilerchamber 20. The diaphragm 25 is held in place at its periphery in thecollar 5 where the rear housing 3 is crimped to the front housing 4. Thediaphragm is a flexible material, such as an elastomer, that isdisplaced when the pressure within the boiler chamber 20 increases.

The boiler chamber may also be defined by an optional liner 26 thatinsulates the rear housing 3 from electrical contact with the heater 21.The liner 26 may be a high temperature plastic or a resinous materialthat can insulate the metal housing.

The front housing 4 encloses a rear part 30 of a shaft assemblyincluding the shaft 10 that extends out of the front housing 4. Theshaft assembly includes a lead screw 30 that is disposed within thefront housing 4 and on which external threads are formed. The shaft 10and lead screw 30 are preferably unitary. Those threads on lead screw 30engage complementary internal threads on a piston nut 31 disposed withinthe front housing 4. At least one of the lead screw 30 and the pistonnut 31 is made of a material, such as a fluorocarbon resin, that has alow coefficient of friction so that the lead screw 30 can easily rotatewithin and relative to the piston nut 31. The shaft assembly includes ahelical spring 32 disposed partially within a recess within the leadscrew 30. The ends of the helical spring 32 are attached to the shaft 10and the piston nut 31, respectively.

The piston nut 31 includes an external shoulder 33 that engages aprotruding guide 34 in the front housing 4. The shoulder and guideprevent rotation of the piston nut 31 relative to the housing 2 but donot prevent linear motion of the piston nut 31 within the front housing4. Where the lead screw 30 and the shaft 10 unite, within and adjacentto the front housing 4, the shaft assembly includes a shoulder 35 thatbears on the inside surface of the front section 4 of the housing. Awasher 36 of a material having a low coefficient of friction, such as afluorocarbon resin, is preferably interposed between shoulder 35 and theinside surface of the front housing 4 to reduce friction as the shaft 10rotates. The portion of the shaft 10 protruding from the front housing 4has a generally circular cross-section but includes two parallel planarsurfaces 37 and 38 for engaging another object and rotating the objectwhen the shaft 10 rotates. Planar surfaces 37 and 38 are only oneembodiment of an engagement means that provides positive transmission ofa rotary mechanical force when the shaft 10 rotates and many alternativearrangements, including a key and groove, gear teeth, planar surfacesforming a square or hexagon in cross-section, and the like may also beused, as readily apparent to a person of skill in the art.

When an appropriate electrical current flows through leads 6 and 7, theheater 21 warms the working fluid within the boiler chamber 20, causingthat fluid to expand and become a gas. The resulting increased pressureurges diaphragm 25 toward the front housing 4. In response to thatforce, the piston nut 31 is urged toward the front housing 4, restrainedby frictional forces and the force applied by the spring 32. When thoseresisting forces are overcome, the piston nut 31 slides linearly towardthe front housing 4 because it is prevented from rotating by theshoulder 33 and the guide 34 of the front housing 4. The sliding causesrotation of the lead screw 30 within the piston nut 31 so that the shaft10 is rotated, as indicated in FIGS. 3(a) and 3(b). In the illustratedembodiment, the rotated position of the lead screw 30 and the shaft 10are shown as a ninety degree rotation from the return position. Theangle of rotation in a particular embodiment of the invention dependsupon the size of the actuator, the amount of displacement and/orexpansion of the diaphragm 25, and the pitch of the complementary,engaging threads of the lead screw 30 and the piston nut 31. Rotationsof less than or more than ninety degrees can easily be achieved.

As shown in FIG. 3(a), when the shaft 10 has been rotated, the spring 32is compressed and applies both a linear and a torsional force, urgingthe shaft 10 toward the return position, i.e., the position shown inFIG. 2(a). However, as long as the pressure in the boiler chamber 20 ismaintained so that the diaphragm 25 is displaced and/or distended, theposition of the shaft 10 shown in FIG. 3(a) is maintained. When theworking fluid in the boiler chamber 20 is allowed to cool, by reductionor removal of the electrical driving current applied to the leads 6 and7, the pressure decreases, the force applied by the diaphragm 25decreases, and the force applied by the spring 32 overcomes the forceapplied by the diaphragm 25. As the pressure declines, the shaft 10counter-rotates to the return position shown in FIG. 2(a) from therotated position shown in FIG. 3(a).

The embodiment of the invention shown in FIG. 2(a) includes the returnto spring 32 urging the shaft 10 toward the return position. When theshaft 10 is urged toward the return position by the mechanical loadapplied to and actuated by the shaft 10, no return spring is needed.

The embodiment of the invention shown in FIG. 2(a) is merelyillustrative of a rotating electrothermal actuator incorporating anon-rotating piston nut and a shaft assembly including a lead screwthreadedly engaging a piston nut and a return spring for urging therotating shaft to return from a rotated position to the return position.Non-helical return springs and return springs mounted on the outside ofthe housing of the actuator may be employed in the invention to producethe same result. The complementary guide 34 and shoulder 33 are only anexample of a means for preventing rotation of the piston nut 31 relativeto the front housing 4. Other arrangements of keys and grooves and othercomplementary elements that prevent rotation will be apparent to thoseof skill in the art. The external/internal threading of the lead screw30 and the piston nut 31 may be reversed.

Another embodiment of the invention is shown in cross-sectional andfront end views in FIGS. 5(a) and 5(b). Like elements in FIGS. 2(a) and5(a) are given the same reference numbers and do not need additionalexplanation. The shaft assembly housed within the front housing 4 issignificantly different in the embodiment shown in FIG. 5(a) compared tothe embodiment shown in FIG. 2(a).

The shaft assembly within the front housing 4 of FIG. 5(a) includes acam driver 40 on which the diaphragm 25 bears and a cam 41 from whichthe shaft 10 protrudes out of the front housing 4. The cam driver 40 andcam 41 are separated by and in contact with a plurality of sphericalmembers 43, such as ball bearings, only one of which is shown in FIG.5(a) for clarity. Cam driver 40 and cam 41 include, on their opposingfaces, variable-depth recesses of the same sense, with regard to theirdepth gradients, that receive the spherical member 43 as described belowin connection with FIGS. 6(a), 7(a), and 7(b). Ball bearings 44 areinterposed between the inside of the front housing 4 and the shaftassembly, i.e., at the rear of the cam 41. A coil spring 45 is disposedon the outside of the front housing 4 with one end of the springattached to the front housing 4 and the other end attached to the shaft10 to apply a biasing force, urging the shaft 10 toward a returnposition. That return position is illustrated in FIGS. 5(a) and 5(b). Asdiscussed above, the spring 45 is not needed when the load actuated bythe shaft 10 urges the shaft toward the return position. The fronthousing 4 includes a guide 34 engaging the shoulder 33 on the cam driver40 so that the cam driver 40 may slide linearly within the front housing4 but may not rotate relative to the front housing. The shaft 10includes a single planar surface 37 for engaging a complementary featureon a driven member engaged by the shaft.

FIGS. 6(a) and 6(b) illustrate the positions of the elements within theactuator of FIG. 5(a) when the shaft 10 has fully rotated. In theembodiment illustrated, the shaft 10 rotates ninety degrees when anelectrical current of sufficient magnitude flows through the heater sothat the working fluid in the boiler chamber 20 becomes a gas, and thediaphragm 25 is fully distended. In order to understand the mechanism ofturning the shaft, it is important to refer to FIG. 7(a) whichillustrates one embodiment of a face 51 used in a cooperating cam driver40 and cam 41. In the illustrated embodiment, each of these faces 51includes three generally kidney-shaped recesses 53 generally lying alonga circumference of a circle centered on the axis of rotation of theshaft 10 and cam 41. Each of the recesses has a depth that varies alongthe circumference of the circle. Generally, each of the recesses 53 oneach of the faces 51 are identical. The widest section of each recess isalso the deepest portion and the narrowest portion is the shallowestsection.

When a force is applied to the cam driver 40 by the diaphragm 25 fromincreasing pressure in the boiler chamber, the cam driver 40 attempts toslide forward, i.e., toward the shaft 10, and is restrained fromrotating relative to the housing 4 by the shoulder 33 and the guide 34.To relieve the force and allow the cam driver 40 to move forward, thecam 41 rotates so that the deepest portions of the opposed pairs ofrecesses of the cam driver 40 and cam 41, respectively, are disposedopposite each other, as shown in FIG. 6(a).

Initially, as shown in FIG. 5(a) in the return position, the shallowerportions of those opposed recesses are opposite each other with thespherical member 43 positioned in the shallower portions of therecesses. In order to continue to accommodate that spherical member 43and to relieve the pressure applied by the diaphragm 25 when the workingfluid is in the gas phase, through a rolling movement of the sphericalmembers, the shaft assembly rotates, bringing the spherical member 43into the deepest portions of the opposed recesses 53, as shown in FIG.6(a). As shown in FIG. 6(b), that rotation of cam 41 causes a ninetydegree counterclockwise rotation of the shaft 10. The ball bearings 44reduce the friction between the shaft assembly and the housing,facilitating the rotation of the shaft 10. The rotation of the shaft 10increases the force applied to the shaft 10 by the spring 45 thatattempts to restore the shaft 10 to the return position. When the forceexerted by the diaphragm 25 is relieved by a reduction of pressure inthe boiler chamber 20, the spring 45 restores the shaft 10 to the returnposition.

The recesses 53 of the face 51 illustrated in FIG. 7(a) result in aparticular direction of rotation, e.g. clockwise, of the shaft 10. Acounterclockwise rotation can be achieved if the sense, i.e. thedirection of depth variation of the recesses, is reversed. For example,a face 51' including recesses 53' of the reverse sense are illustratedin FIG. 7(b). When recesses 53' are employed on the faces of cooperatingcam drivers and cams, a direction of rotation opposite to that achievedwith the faces 51 of FIG. 7(a) is produced.

The cooperating faces cam driver 40 and the cam 41 illustrated in FIGS.7(a) and 7(b) are merely illustrative. A larger or smaller number ofrecesses can be included in each face of the cam and cam driver,although each of the faces preferably includes at least two recesses andtwo spherical members. A single recess on each of the cam driver and camfaces may be employed if a means, such as a protrusion from one of thefaces, is provided to prevent binding between the housing and cam driverand/or cam during rotation or counter-rotation of the shaft. Therecesses may be longer or shorter along the circumference of the circledepending upon the angle of rotation between the rotated position andthe return position of the shaft. That angle of rotation may be greateror less than ninety degrees.

Yet another embodiment of the invention is shown schematically in FIG. 8in a cross-sectional view. In that actuator, the guide for preventingrotation of the cam driver is a pin 60 attached to the cam driver thatengages a slot in the front housing 4. In this embodiment, no protrudingor asymmetrical guide is required on the front housing to engage atongue on the cam driver. In addition, the cam driver includes aperipheral recess for receiving a folded or rolled annular peripheralportion of the diaphragm 25 which is not stretched taut as in theembodiment of FIG. 5(a). In FIG. 8, the diaphragm 25 is even larger,relative to the axial cross-sectional area of the actuator, than thediaphragm in the embodiment of the invention shown in FIG. 2(a). Unlikethe relatively taut diaphragm of the embodiment of FIG. 5(a), in FIG. 8,the diaphragm 25 "rolls" as the cam driver 40 slides. Of course, any ofthe diaphragm arrangements can be used with any of the actuatorembodiments of the invention described here. The embodiment of FIG. 8,like the embodiment of FIG. 5(a), includes spherical bearings 44disposed between the cam and the housing for reducing the rotationalfriction of the shaft assembly in the housing. While two such ballbearings are illustrated in the cross-sectional view shown, a largernumber of ball bearings can be employed. Alternatively, the ballbearings 44 of the embodiment of FIG. 5(a) and the washer 36 of FIG.2(a) can be interchanged or a different friction-reducing bearing can beemployed.

We claim:
 1. An electrothermal actuator for converting electrical energyto rotational mechanical motion comprising:a housing; a flexiblediaphragm disposed in the housing and defining a boiler chamber withinthe housing; a working fluid disposed within the boiler chamber,changing phase in response to heating and thereby displacing thediaphragm; heating means disposed within the boiler chamber for heatingthe working fluid in response to an electrical current flowing throughthe heating means; and a rotatable shaft assembly partially disposedwithin the housing and including a rotatable shaft extending outside thehousing and rotating between a return position and a rotated position, athreaded piston nut slidingly disposed within the housing, a lead screwrotatably disposed within the housing and attached to the shaft,threadedly engaging the piston nut, whereby the shaft and lead screwrotate from the return position toward the rotated position in responseto heating of the working fluid.
 2. The actuator of claim 1 wherein thepiston nut and the housing include means for preventing rotation of thepiston nut relative to the housing.
 3. The actuator of claim 2 whereinthe means for preventing rotation comprises a guide on the housing and ashoulder on the piston nut engaging the guide.
 4. The actuator of claim2 including a return spring engaging the shaft and urging the shaft torotate toward a return position.
 5. The actuator of claim 4 wherein thelead screw includes a recess and the return spring is disposed partiallywithin the recess and engages the piston nut and the shaft.
 6. Theactuator of claim 1 including means for reducing friction disposedbetween and bearing on the housing and the lead screw.
 7. The actuatorof claim 6 wherein the means for reducing friction comprises a washerhaving a low coefficient of friction.
 8. An electrothermal actuator forconverting electrical energy to rotational mechanical motioncomprising:a housing; a flexible diaphragm disposed in the housing anddefining a boiler chamber within the housing; a working fluid disposedwithin the boiler chamber, changing phase in response to heating andthereby displacing the diaphragm; heating means disposed within theboiler chamber for heating the working fluid in response to anelectrical current flowing through the heating means; and a rotatableshaft assembly partially disposed within the housing and including arotatable shaft extending outside the housing and rotating between areturn position and a rotated position without translational movement ofthe shaft relative to the housing, a threaded piston nut slidinglydisposed within the housing, a lead screw rotatably disposed within thehousing and attached to the shaft, threadedly engaging the piston nut,whereby the shaft and lead screw rotate from the return position towarda rotated position in response to heating of the working fluid.
 9. Theactuator of claim 8 wherein the piston nut and the housing include meansfor preventing rotation of the piston nut relative to the housing. 10.The actuator of claim 9 wherein the means for preventing rotationcomprises a guide on the housing and a shoulder on the piston nutengaging the guide.
 11. The actuator of claim 9 including a returnspring engaging the shaft and urging the shaft to rotate toward thereturn position.